This invention relates to the efficient use of a fan for cooling, and, more particularly, to a cooling fan which operates intermittently and in response to a threshold temperature.
Typically, the least expensive apparatus for cooling an electric motor, internal combustion engine or the like is a cooling fan driven constantly from the shaft of the motor or engine. One problem with driving a fan constantly is that it takes energy to drive a fan. Since, in many applications it is not necessary to cool an apparatus constantly and, accordingly, not necessary to drive the fan constantly, it is desirable to drive the fan intermittently to conserve energy.
One method of driving a fan intermittently is to drive the fan through an electrically operated clutch. This type of clutch can be activated by means of a control circuit which senses the temperature of the cooled apparatus and engages or disengages the clutch accordingly. One problem with this type of fan operation is the cost of this type of clutch and the corresponding control circuit. For small motors and engines the amount of energy savings due to intermittent fan operation may not justify the cost of an electric clutch and control circuit.
Another problem with intermittently driven fans is the accurate detection of the temperature of the component for which the temperature must be regulated. For example, in an electric motor the rotor temperature may be the temperature which must be regulated. The temperature of the rotor could be measured indirectly from the temperature of a stationary part of the motor, or measured directly. One problem with indirect measurement is that an increased rotor temperature may lead the measured temperature excessively, resulting in thermal damage during the lead time. One problem with direct measurement is transferring the signal from a sensor, such as a thermocouple, on a moving element, such as a rotor, to a control circuit. Methods for this transfer, for example slip rings, are available but these methods are expensive and may not be reliable for the life of a motor, which in many cases is over ten years.